On hybrid vehicles or electric vehicles attracting attention in recent years, a motor is mounted as a motive power source, and a battery supplying the motor with electric power is mounted. As such a battery mounted on the hybrid or electric vehicles, a secondary battery that can repeatedly be charged and discharged, such as a nickel hydride battery or a lithium ion battery, is used.
The battery mounted on the hybrid or electric vehicles is adapted to high voltages so as to correspond to the increased output of the motor. Generally, the secondary battery is configured as a battery pack formed by a plurality of serially connected battery modules each formed by a plurality of serially connected battery cells.
In many cases, a monitoring apparatus that senses abnormality by monitoring a plurality of cells constituting such a battery pack (assembled battery) is provided along with the assembled battery. For example, an abnormality determining apparatus disclosed in Japanese Patent Laying-Open No. 2005-114401 determines abnormality of an assembled battery based on a value of a voltage sensor provided for each battery module. Based on usage frequency of a plurality of battery modules, the abnormality determining apparatus sets a threshold value for determining abnormality of the battery modules. Therefore, a replaced battery (new battery) is prevented from being erroneously determined as an abnormal battery.
In some cases, a high-voltage secondary battery used for hybrid vehicles or the like is configured so that the number of battery cells constituting a battery module is different among a plurality of battery packs, in consideration of the limited installation space. On the other hand, the aforementioned Japanese Patent Laying-Open No. 2005-114401 does not particularly disclose about whether or not the abnormality determining apparatus can appropriately monitor the secondary battery when there are battery modules being different in the number of battery cells.